In gas turbine systems for generating power, feed air is compressed and combusted with a reactant to raise its temperature, and subsequently expanded through a turbine to produce power. Oxygen producing equipment has been combined with some of these gas turbine systems to produce oxygen at an incremental cost. Gas turbine power systems have also been combined with steam power generating systems to generate additional power, where the expanded hot gas may also be used to generate steam.
One type of oxygen producing equipment utilizes solid electrolyte ion transport membrane. The ion transport system operates at a significantly higher temperature, in the range of from about 500.degree. C. to about 1200.degree. C., than the compressor discharge of a gas turbine system, whose operating temperature rarely reaches 375.degree. C.
There are now two types of solid electrolyte ion transport membranes under development. They include ionic conductors that conduct only ions through the membrane and mixed conductors that conduct both ions and electrons through the membrane. An ion transport membrane exhibiting mixed conduction characteristics can transport oxygen when subjected to a ratio of partial pressures of oxygen across the membrane without the need for an applied electric field or external electrodes which would be necessary with ionic only conductors. As used herein, the terms "solid electrolyte ion transport system", or simply "solid electrolyte" or "ion transport membrane" is used to designate either a system using an ionic-type (electrically-driven) system or a mixed conductor-type (pressure-driven) system unless otherwise specified.
Mixed conductors are materials which, at elevated temperatures, contain mobile oxygen-ion vacancies that provide conduction sites for selective transport of oxygen ions through the material. The transport is driven by the ratio of oxygen activities, i.e., oxygen partial pressures (P.sub.O2) across the membrane, as oxygen ions flow from the side with higher partial pressure of oxygen to that with lower partial pressure of oxygen. Ionization of oxygen molecules to oxygen ions takes place on the cathode-side (or the retentate zone) of the membrane. The oxygen ions recombine on the permeate zone of the membrane giving up electrons. For materials that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the electrolyte and the electrons are returned to the cathode in an external circuit. In mixed conducting materials, electrons are transported to the cathode internally, thus completing the circuit and obviating the need for external electrodes. It is believed that the reaction of the permeated oxygen with fuel takes place on the surface or in the boundary layers rather than in the bulk phase on the anode-side (or the permeate zone).
Partial oxidation reactions (POx) involving carbonaceous feedstock are common methods for producing synthesis gas. Partial oxidation is also used to produce ethylene oxide, acrylonitrile and other chemicals. Synthesis gas, comprised of carbon monoxide and hydrogen, is a valuable industrial gases and important precursors for production of chemicals including ammonia, alcohols (including methanol and higher carbon alcohols), synthesis fuels, aldehydes, ethers, and others. Feedstocks including natural gas, coal, naphtha, and fuel oils are commonly used to produce synthesis gas by partial oxidation or steam reforming reactions. The partial oxidation reactions may be further represented as follows: EQU C.sub.m H.sub.n +m/2 O.sub.2 =m CO+n/2 H.sub.2,
where C.sub.m H.sub.n is a hydrocarbon feedstock.
To a minor degree, steam reformation may also take place, as is represented as follows: EQU C.sub.m H.sub.n +m H.sub.2 O=m CO+(m+n/2) H.sub.2,
where C.sub.m H.sub.n is a hydrocarbon feedstock.
Conventional POx processes frequently use oxygen molecules produced by traditional gas separation processes (for example, pressure swing adsorption, cryogenic distillation) that typically operate at temperature below 100.degree. C. Since POx itself typically requires a high temperature of operation of more than 800.degree. C., integration between partial oxidation reaction and traditional oxygen separation is not realized by the conventional process. As a result, conventional partial oxidation has often been characterized by low feedstock conversion, low hydrogen to carbon monoxide ratio, and low hydrogen and carbon monoxide selectivities. Additionally, the external oxygen supply typically required in a partial oxidation reaction adds significantly to capital and operating costs, which may amount to as much as 40% of the total synthesis gas production cost.
It should be noted that the use of a solid electrolyte membrane for POx in an electrochemical reactor has been disclosed in U.S. Pat. Nos. 5,160,713 and 5,306,411, both to Mazanec et al., but neither of these patents disclose processes to produce an oxidized product in conjunction with a synergistic use of a gas turbine system.
Two of the most attractive features of the ion transport membrane system are the membrane's infinite selectivity for oxygen transport and its ability to transport oxygen from a low pressure stream to a high pressure stream as long as a ratio of partial oxygen pressure of greater than 1 exists, as is the case when the permeated oxygen reacts with a fuel gas. For the purpose of this invention, ion transport membrane materials that transport oxygen ions are deemed useful for the separation of oxygen from oxygen-containing gas mixtures. The types of membrane materials proficient in transporting oxygen ions are discussed in commonly assigned U.S. patent application Ser. No. 08/490,362, entitled "Method for Producing Oxygen and Generating Power Using a Solid Electrolyte Membrane Integrated with a Gas Turbine", filed Jun. 14, 1995, and concurrently filed application Ser. No. 08/848,200, entitled "Method of Producing Hydrogen Using Solid Electrolyte Membrane", which are incorporated herein by reference.
U.S. patent application Ser. No. 08/490,362 discloses methods for utilizing high combustor temperatures reached by a power generation system to drive an oxygen production system at acceptable operating temperatures for both systems. That application also discloses a method which efficiently produces both oxygen and power as products. U.S. Pat. Nos. 5,516,359, 5,562,754, 5,565,017 and European Patent Publication No. 0 658 366 produce oxygen in processes that are integrated with a gas turbine.
The efficient use of ion transport systems to produce other chemical gas products in conjunction with gas turbine power generating capacities is not believed to have been previously realized. Although the concept of integrating an air separation unit with gas turbine systems are known, there has not believed to have been synergistic use of energy integration between the air separation unit wherein oxidized products are produced in conjunction with gas turbine systems with which an ion transporting oxygen separating membrane is integrated.